trouble setting idle speed below 1200rpm

[fredmeister]

I have been bumping my initial static timing up from stock settings in order to improve the performance of my 79 930 at lower rpms since we all know the terrible low rpm response of our cars when set to factory specs.

Anyway, I have removed most all the idle speed and other air control devices and am running without the AAR and plugged the ports off the intercooler that would have went to this.

When i advance the timing to anything above 10 degrees BTDC I cannot get the idle speed down any lower than 1100rpm. At 12 degrees BTDC it is almost 1200rpm. The idle speed adjustment screw is completely closed at this point so I am not sure what else I can do to control the idle speed.

I would like to get it down to 950-1000rpm but how can I do this if the idle speed control screw is bottomed down tight?

Anyone else have this issue?

I dont think there is a screw adjustment on the throttle plate stop is there?

I also noticed my throttle plate has a small hole (3mm or so) drilled into its surface which would allow a small amount of air bypass....IS THIS NORMAL?

Did some searching on the forum and I still have the Decel valve hooked up. Noticed some suggested this might be bypassing air. My vacuum at idle varies between 6.5 to 8psi absolute pressure which might be triggering the Decel valve....is this right? Then this might be the problem from what I read.

If I disconnect the small vacuum line to the front of the Decel valve canister and leave the other air hoses connected will this take the can out of the loop and deactivate the Decel valve? Not sure how this canister works.

Thank in advance,
Fred

[Crewchief]

My 79 930 did the same thing. I plugged the hole in the throttle with a small bolt and nylock nut with locktight on it. Idle was then able to adjust with air bypass screw.

[stevie 77 930]

When working correctly, the decel valve (vacuum limiter) remains closed and requires about 18 inchs of vacuum to open.

[xbmwguy]

there is an stop screw adjustment on the side of the t body near the linkage. you can loosen it without removing the t body but it is a pain in the butt. a little goes a long way

[JFairman]

I would plug that hole in the butterfly and try adjusting the little stop screw too.

[jwasbury]

The hole in my throttle butterfly has been plugged (looks like solder). It was that way when I bought it. I run 10+ degrees BTDC with twin plug and have no issues keeping the idle rpms below 1000

[Speedy Squirrel]

Originally Posted by fredmeister

I also noticed my throttle plate has a small hole (3mm or so) drilled into its surface which would allow a small amount of air bypass....IS THIS NORMAL?

The early US cars ('77 to '79) have the hole in the throttle plate. The later US cars ('86 to '89) do not.

[fredmeister]

Just an update.

I removed the intercooler and put a piece of silver metal aluminum tape over the hole in the throttle plate.

That did the trick....now I can control idle speed via the air bypass screw once again.

Why the hell did Porsche put that hole in there in the first place? PITA.

Fred

[mooney265]

The metal tape was a good idea!! I had the same issues with my idle, so I had mine soldered/braised up by a radiator repair man [like jwasbury]. Would have gladly used the tape.

The reason I didn't do the bolt/nut thing was I was afraid of hampering airflow during WOT. Was I correct in that?

[fredmeister]

Originally Posted by mooney265

The metal tape was a good idea!! I had the same issues with my idle, so I had mine soldered/braised up by a radiator repair man [like jwasbury]. Would have gladly used the tape.

The reason I didn't do the bolt/nut thing was I was afraid of hampering airflow during WOT. Was I correct in that?

Exactly that is why the metal tape was on my mind. Plus with a screw and nut there is always the possibility of it coming loose and passing thru an intake valve.

The tape presents no cross section to impede air flow. Plus I did not have to remove the brass throttle plate to make the fix.

see ya
Fred

[mark houghton]

Originally Posted by fredmeister

Exactly that is why the metal tape was on my mind. Plus with a screw and nut there is always the possibility of it coming loose and passing thru an intake valve.

The tape presents no cross section to impede air flow. Plus I did not have to remove the brass throttle plate to make the fix.

see ya
Fred

Can you say....JB Weld? Just drop'll do ya!

[Ronnie's.930]

Originally Posted by mark houghton

Can you say....JB Weld? Just drop'll do ya!

Where's Brando/quattrorunner when we need him?

[cellison]

not to hyjack, but after i drive the car, shut it off and restart immediatly or up to 20-30 minutes of rest, the idle runs around 2-2,500, then settles back down to 900 or so after it is driven a few miles.

[mark houghton]

Originally Posted by cellison

not to hyjack, but after i drive the car, shut it off and restart immediatly or up to 20-30 minutes of rest, the idle runs around 2-2,500, then settles back down to 900 or so after it is driven a few miles.

As annoying as it is, it should do just exactly that.

[JFairman]

It's adjustable too.. the same way the cold start control pressure is adjusted.

I never see it higher than around 1500 -1800rpms and that's only when it's cold out and the car has been shut off for around a half hour after being driven for a while and it's up to operating temp.

In that case the cylinders, heads, and oil are still very hot and don't need any CIS cold start crap going on but the cold start aux air valve has cooled off and it's bimetalic spring has opened it's air valve letting too much metered air bypass the throttle body and then the idle speed is way too high until the heater element heats up the bimetallic spring and shuts the air valve.

[mooney265]

Originally Posted by JFairman

It's adjustable too...

How is it adjustable? I removed mine some years back, but have been considering putting it back on. Just don't like the 2K+ rpm cold starts. I realize I could lessen the diameter of the hose and restrict airflow, but I'm figuring you know a way to actually adjust the thingy??? Thanks... Shannon

[JFairman]

When you have it in your hand and look at it you'll see a round steel slug pressed into the cast aluminum housing similar to the one pressed into the top of the CPR or WUR.

Just like in the WUR the bimetallic spring with heater element is mounted to it inside the housing and repositioning the interference fit steel slug with a drift punch and small hammer or C clamp moves the position of the bimetallic spring which will change how far the air valve opens when cold.
The more that valve is open the more air is allowed to bypass the throttle body and idle speed rises.
It's hard for me to describe.. but if you drill out the aluminum rivets that hold the aux air valve housing together you can take it apart and you'll see how it works. It's really simple inside.

Since you have it off the car already you should take it apart and you'll see how it works and how adjusting the position of the steel slug will adjust how far the valve opens when cold or closes when it warms up.
Then you put it back together with small bolts or phillips head screws and nylock nuts where the rivets were. Stainless ones look nicest and don't corrode.

[cellison]

Originally Posted by mark houghton

As annoying as it is, it should do just exactly that.

well that's good to know albiet it IS annoying. any thoughts why its that way?

[JFairman]

There is a simple way to gain manual control over it if you like...

It is that way because CIS mechanical fuel injection has no engine management ECU or computer that monitors idle speed and controls an electronic idle control valve that bypasses air around the throttle body to maintain a steady idle speed at all times including when an AC compressor is engaged.

I got sick of this annoyance in so I figured out a way to manually control the airflow being bypassed around the throttle body for high idle during cold starts.
I used a heater valve from an old CIS BMW 320i I pulled off a car in a junk yard and a generic choke cable from an auto parts store to control it and it works perfect.

The water cooled Porsche 924 uses the same heater valve and pelican sells new ones here for about $22. Pelican Parts - Automotive Parts and Accessories - Porsche & BMW

I installed the heater valve in the air hose going from the cold start high idle auxillary air valve to the fitting on the intake manifold under the throttle body. It fits perfect there and then I ran the generic push pull control cable out of the engine compartment over the back of the engine tin and then around over the left side of the torsion bar tube under the car and into the car right under the left rear seat cushion so the little aluminum pull knob on the end of the cable is easy to reach from the driver seat with my left hand and it works smooth and great. I love it.
Maybe I should take pics of it and post it later.

In Florida the only time I use it is on cold days in the winter.

[JFairman]

Here's pelicans pic of the 924 heater valve I mentioned above.
The one I got off an old BMW 320i in a junk yard has a little metal clip on the end for mounting and holding the operating cable in place and then I used needle nose pliers to make a little bend in the end of the cable where it goes through the little hole on the end of the white plastic control lever.
A small hose clamp around the end of the black plastic arm on it would also probably work to hold the cable in place. I don't see the little metal cable retaining clip on the end in this pic of the valve.